July-August 2009

Harnessing the Sun: The Promises of Solar Energy

by Jan Null

The world’s increasing need for clean, renewable energy has given greater momentum to efforts to build the next generation of solar-powered devices for both small- and large-scale use. In a world where experts say humans need to reduce their carbon footprint, solar energy is a win-win proposition. Not only is it an inherently clean technology, but every megawatt of power produced by solar energy is one less that needs to be produced by fossil fuel.

Solar Power in HistoryHarnessing the sun for energy is not a new idea by any means. As early as the fourth century B.C.E. the Greek philosopher Socrates set forth a number of principles for passive solar design, including orienting buildings in ways to utilize the sun’s heat in the winter and to minimize its impact in the summer.

The first century AD saw the Romans expand the use of solar energy to heat their buildings and large central baths, thereby lessening the need to cut down forests for firewood. At the same time, the Romans also recognized that they could use glass, which had been invented recently, to build sun-warmed greenhouses.

In 1515, Leonardo da Vinci sketched plans for a parabolic mirror that could be used to concentrate the sun’s energy to heat a boiler. This type of device was put into use 2 centuries later, when a solar furnace was used to melt platinum at around 3200°F (1800°C).

It wasn’t until 1860, during the Industrial Revolution, that French mathematician Auguste Mouchout expressed strong concerns about Europe’s growing dependence on coal, and over the next decade he went on to develop the first solar motor. Mouchout’s device tracked the sun and focused its rays onto a boiler assembly to produce a small steam engine. Modifications and refinements to Mouchout’s work continued well into the twentieth century and laid the foundation for the largest commercial sector of solar energy today—concentrated solar power (CSP). There continues to be accelerated growth in CSP, with larger power-generating facilities routinely making news.

Meanwhile, the discoveries that led to the eventual conceptualization of photo-voltaic cells (PVCs)—what would become the other major means for producing solar energy—also began in the late nineteenth century. The foundation for these cells began with the discovery of the element selenium by Swedish chemists Jakob Berzelius and J. G. Gahn in 1818. A number of scientists found that one of the properties of selenium was its mysterious ability to produce small amounts of electricity when exposed to light. It wasn’t until 1904 that Albert Einstein wrote a paper explaining this photoelectric effect—work that led to his Nobel Prize nearly 20 years later. However, it would be another 50 years before Bell Labs produced a silicon solar cell capable of absorbing photons from the sun and directly converting these into electricity that could power everyday electrical devices. Photovoltaic technology has continued to evolve, from a variety of silicon-based cells to more efficient polycrystalline materials. In terms of small-scale application of solar energy, PVCs have become ubiquitous, seen on everyday devices like calculators, highway signs, and patio lights.

These two methods for harnessing the sun’s energy—concentrated solar power and photo-voltaic cells—make up today’s two major means for generating solar power. CSP is used primarily on a larger scale to generate megawatts of energy from big power facilities. Conversely, PVCs are mostly used in the continued growth of residential and small-scale applications, as evidenced by more and more solar panels sprouting from rooftops.

Small-Scale Solar PowerIn the residential sphere, which in the modern age is dominated by the use of PVCs, solar energy has been employed to heat homes both actively and passively for centuries. Socrates’ passive solar designs, mentioned earlier, are some of the oldest examples of this. As early as 1930, roof-mounted solar hot-water heaters reduced the amount of non-renewable energy needed for homes and swimming pools, especially in the sunnier climates of California and Florida. The water in these systems is heated as it passes through a series of roof-mounted tubes. Today, there is an installed base of about a million and a half homes with solar-heated water for domestic use and swimming pools.Small-scale PVC installations have benefited from improved technology, as well as an array of federal, state, and local incentives. The passage of the Emergency Economic Stabilization Act of 2008 (EESA2008) included a 30 percent tax credit for the installation of solar electric systems. The combination of these federal incentives and myriad state programs translates into a “typical” $40,000 home solar system costing closer to $25,000. And as residential and small-business solar systems get even more efficient, it is projected that the current 10-15 year period it takes for a solar system to pay for itself will drop to somewhere between 7-10 years.

Solar power from PVCs can also be combined with other renewable technologies for small-scale use, resulting in such innovations as the Solar Prius automobile. Currently, a $3,500 solar-panel home kit from Solar Electrical Systems is available for consumers to install in place of the standard roof of a Prius. It charges the vehicle’s high voltage and supplemental batteries for an additional 20 miles of range in electric mode. In 2010, a similar Toyota factory-installed solar-panel option will be available. In addition to retrofitted cars with solar panels, a number of prototype vehicles have set distance and mileage records using solar power. In March of 2009 the solar powered vehicle Power of One, resembling a flying saucer on 3 wheels, completed a world record 12,5000-mile odyssey around North America running only on sunshine.

And far above the ground, several odd-looking solar-powered aircraft have been pushing the envelope in time aloft and distance records. As early as 1981, the Solar Challenger, equipped with PVC panels supplying a maximum power of 2.5 kilowatts (kW) to its electric engines, succeeded in crossing the English Channel. Plans are currently in the works for a Swiss-based team to fly the Solar Impulse around the world. The Solar Impulse has a wingspan of more than 200 feet, weighs only 3,300 pounds, and will be able to stay aloft for 36 hours at a time and cruise at 28,000 feet.

Even higher in the sky, above the earth’s atmosphere, solar panels are the main power source for many satellites, as well as for the International Space Station. They build on the legacy of Vanguard 1, the first solar-powered satellite, which was launched in March 1958 at the beginning of the Space Race.

Across the country, the greatest market penetration for residential and small business PVC solar energy has been in the sunnier southern tier, especially in the Southwest. California has led the latest solar stampede with its Solar Initiative and goal of “a million solar roofs” within the next decade. This initiative provides rebates and incentives for builders and homeowners to add solar energy to existing and new houses and businesses. Specific elements of the initative have builders of subdivisions offering solar energy as an optional feature and public utilities “buying back” excess electricity generated by homeowners. Nationally, the Department of Energy has recognized 25 Solar America Cities for their innovation and for “working to accelerate the adoption of solar energy technologies for a cleaner, more secure energy future.” Many of these cities are in sunny locales, but innovative programs in places like Seattle, Minneapolis-St. Paul, Pittsburgh, and Boston have put these cooler cities on the list.

Large-Scale Solar InstallationsWhile the PVC industry is benefiting from the production of lighter, more durable, and more efficient solar panels, the CSP industry is seeing its greatest growth in the improved integration of large-scale technology. The most significant advances over the past five years, and the area in which the most important advances are projected to happen, revolve around this utility-scale production of electricity from the sun’s energy.

CSP plants are conceptually similar to Mouchout’s “solar motor” of 150 years ago, with a fluid that is superheated by the sun and becomes the steam that turns a turbine for electricity. There are two major design types for CSP facilities. In one of them, water flows through a closed loop, which takes it through or near solar collectors, thereby heating the water along its way back through a turbine. The other configuration type has a central tower, which houses mirrors that are focused on the fluid flowing through it. These towers can be 300-600 feet high, and their fluids can be superheated in excess of 2200°F. These designs can use mirrors that track the sun, parabolic mirrors that focus the sun’s rays, or both.

The lion’s share of large-scale solar-generating capacity, both current and proposed, is from CSP plants, beating out PVC plants by a ratio of approximately 5:1. There are already CSP plants that generate 300 megawatts (MW), including a CSP complex near Seville, Spain, which is producing 310 MW. Within the next 2 years, a number of500-MW facilities will begin generating electricity in the United States, Europe, and Australia. By 2016, a 1.3 -gigawatt (GW) complex will be in operation in the Mojave Desert of California with enough power for 845,000 homes.

Though large-scale output of solar energy is more commonly produced by CSP plants, PVCs can also produce significant amounts of electrical energy from the sun if used in vast numbers. A critical factor is the PVC’s efficiency at converting the sun’s shortwave energy waves to electricity. The simple monocrystalline silicon cells of 50 years ago, whose efficiency was on the order of only 5 percent, have evolved into exotic multicrystalline arrays with efficiencies approaching 40 percent. In addition to this greatly increased efficiency, PVCs are now lighter, come in a wider variety of materials, and are significantly less expensive to produce. Currently, the world’s largest operational PVC plants are in Spain, with an output of 20 megawatts. (The capacity of power generating facilities, be they solar, wind, or fossil fuel, is measured by the peak amount of power they can put onto the power grid at any given time.) However, in 2011 the record capacity will increase by more than an order of magnitude when a 300-MW plant goes online near Deming, New Mexico. The plant will generate enough power for 250,000 homes.

These advances are the result of more efficient technologies that are available at increasingly competitive costs. Currently, only about 0.7 percent of global electricity is solar, which is roughly half the output created from wind energy. However, the U.S. Department of Energy’s Solar Energy Technologies Program projects going from the current 1 GW of generation capacity to more than 150 GW by 2030.

But with either method of generating solar energy, in order to be successful, solar power will have to overcome several roadblocks, the biggest of which is cost. According to the U.S. National Renewable Energy Laboratory, the consumer cost per kilowatt-hour for PVC-produced electricity fell from $0.40 in 1990 to $0.24 in 2000, and it is currently projected to reach about $0.10 per kWh in 2010. This is still more costly than other renewable energy sources such as wind power. CSP has become somewhat more affordable, falling from $0.20 per kWh in 1990 to about $0.10 in 2000, and by 2010 it is forecasted to be approximately $0.06. These CSP projections bring it close to those for wind energy and approach the current national average for fossil fuel power.

CSP is also at a disadvantage because it requires many hours of direct sunlight and is therefore mostly confined to the southwestern United States. PVCs can operate in indirect sunlight, making this option more viable for the rest of the country. Utility-scale operations of both CSPs and PVCs also require large tracts of land for their mirrors or solar arrays. There is also the need for an improved power grid infrastructure so that the electricity from these remote locations can be transmitted to users in a more cost-efficient manner.

The good news is that the solar industry recently received a boost from the administration of President Barack Obama. In addition to continuing existing incentives for renewable sources of energy, the 2009 Economic Stimulus legislation contained $11 billion to modernize the electrical grid and make it “smart.” The proposed smart grids are digital and more efficient at getting electricity to users from large power plants while also allowing consumers to sell excess power from their residential solar installations back to their utility companies. The stimulus package also included another $6 billion in loan guarantees for renewable energy projects. Representatives of the solar industry estimate that this will translate into significant growth for solar power and the creation of more than 100,000 jobs within the next 2 years.

The forecast looks sunny not just for solar energy but also for renewable energy in general. And the once-mocked idea of generating 20 percent of the nation’s power from renewable sources by 2020 appears to be more feasible than ever before. JAN NULL is an adjunct professor of meteorology at San Francisco State University and a Certified Consulting Meteorologist with Golden Gate Weather Services.